Distribution of Cell-Common Downlink Signals in a Hierarchical Heterogeneous Cell Deployment

ABSTRACT

A high-power point and one or more low-power points transmit signals that are associated with the same cell-identifier in a heterogeneous cell deployment. The coverage areas corresponding to the low-power points fall at least partly within the coverage area for the high-power point, so that mobile stations within range of a low-power point are also within range of the high-power point, from a downlink perspective. The same CRS signals are transmitted by both the high-power (macro) point and some or all of the low-power (pico) points. At the same time, the network transmits CRS-based PDSCH for a particular UE on both the high-power point as well as on some or all of the low-power points. In some embodiments only a subset of the points, e.g., those points that the UE hears sufficiently well, participate in the PDSCH transmission using CRS for channel estimation.

RELATED APPLICATIONS

This application claims priority to U.S. patent application Ser. No.13/389,501, filed 8 Feb. 2012, which is a national stage entry under 35U.S.C. §371 of international patent application serial no.PCT/SE11/51307, filed 2 Nov. 2011, which claims priority to U.S.Provisional Application Ser. No. 61/440,916, filed 9 Feb. 2011. Theentire contents of each of the foregoing applications are incorporatedby reference herein.

TECHNICAL FIELD

The present invention relates generally to the control of devices inwireless communication networks, and more particularly relates totechniques for allocating reference signals to carrier resources inthese networks.

BACKGROUND

The 3rd-Generation Partnership Project (3GPP) is continuing developmentof the fourth-generation wireless network technologies known asLong-Term Evolution (LTE). Improved support for heterogeneous networkoperations is part of the ongoing specification of 3GPP LTE Release-10,and further improvements are being discussed in the context of newfeatures for Release-11. In heterogeneous networks, a mixture of cellsof different sizes and overlapping coverage areas are deployed.

One example of such a deployment is seen in the system 100 illustratedin FIG. 1, where several pico-cells 120, each with a respective coveragearea 150, are deployed within the larger coverage area 140 of amacro-cell 110. The system 100 of FIG. 1 is suggestive of a wide-areawireless network deployment. However, other examples of low power nodes,also referred to as “points,” in heterogeneous networks are home basestations and relays.

Throughout this document, nodes or points in a network are oftenreferred to as being of a certain type, e.g., a “macro” node, or a“pico” point. However, unless explicitly stated otherwise, this shouldnot be interpreted as an absolute quantification of the role of the nodeor point in the network but rather as a convenient way of discussing theroles of different nodes or points relative to one another. Thus, adiscussion about macro- and pico-cells could just as well be applicableto the interaction between micro-cells and femto-cells, for example.

One aim of deploying low-power nodes such as pico base stations withinthe macro coverage area is to improve system capacity, by means ofcell-splitting gains. In addition to improving overall system capacity,this approach also allows users to be provided with a wide-areaexperience of very-high-speed data access, throughout the network.Heterogeneous deployments are in particular effective to cover traffichotspots, i.e., small geographical areas with high user densities. Theseareas can be served by pico cells, for example, as an alternativedeployment to a denser macro network.

The most basic means to operate heterogeneous networks is to applyfrequency separation between the different layers. For instance, themacro-cell 110 and pico-cells 120 pictured in FIG. 1 can be configuredto operate on different, non-overlapping carrier frequencies, thusavoiding any interference between the layers. With no macro-cellinterference towards the under-laid cells, cell-splitting gains areachieved when all resources can simultaneously be used by the under-laidcells.

One drawback of operating layers on different carrier frequencies isthat it may lead to inefficiencies in resource utilization. For example,if there is a low level of activity in the pico-cells, it could be moreefficient to use all carrier frequencies in the macro-cell, and thenbasically switch off the pico-cells. However, the split of carrierfrequencies across layers in this basic configuration is typically donein a static manner.

Another approach to operating a heterogeneous network is to share radioresources between layers. Thus, two (or more) layers can use the samecarrier frequencies, by coordinating transmissions across macro- andunder-laid cells. This type of coordination is referred to as inter-cellinterference coordination (ICIC). With this approach, certain radioresources are allocated to the macro cells for a given time period,whereas the remaining resources can be accessed by the under-laid cellswithout interference from the macro cell. Depending on the trafficsituations across the layers, this resource split can change over timeto accommodate different traffic demands. In contrast to the earlierdescribed static allocation of carrier frequencies, this way of sharingradio resources across layers can be made more or less dynamic dependingon the implementation of the interface between the nodes. In LTE, forexample, an X2 interface has been specified in order to exchangedifferent types of information between base station nodes, forcoordination of resources. One example of such information exchange isthat a base station can inform other base stations that it will reducetransmit power on certain resources.

Time synchronization between base station nodes is generally required toensure that ICIC across layers will work efficiently in heterogeneousnetworks. This is of particular importance for time-domain-based ICICschemes, where resources are shared in time on the same carrier.

Orthogonal Frequency-Division Multiplexing (OFDM) technology is a keyunderlying component of LTE. As is well known to those skilled in theart, OFDM is a digital multi-carrier modulation scheme employing a largenumber of closely-spaced orthogonal sub-carriers. Each sub-carrier isseparately modulated using conventional modulation techniques andchannel coding schemes. In particular, 3GPP has specified OrthogonalFrequency Division Multiple Access (OFDMA) for the downlinktransmissions from the base station to a mobile terminal, and singlecarrier frequency division multiple access (SC-FDMA) for uplinktransmissions from a mobile terminal to a base station. Both multipleaccess schemes permit the available sub-carriers to be allocated amongseveral users.

SC-FDMA technology employs specially formed OFDM signals, and istherefore often called “pre-coded OFDM” or Discrete-Fourier-Transform(DFT)-spread OFDM. Although similar in many respects to conventionalOFDMA technology, SC-FDMA signals offer a reduced peak-to-average powerratio (PAPR) compared to OFDMA signals, thus allowing transmitter poweramplifiers to be operated more efficiently. This in turn facilitatesmore efficient usage of a mobile terminal's limited battery resources.(SC-FDMA is described more fully in Myung, et al., “Single Carrier FDMAfor Uplink Wireless Transmission,” IEEE Vehicular Technology Magazine,vol. 1, no. 3, September 2006, pp. 30-38.)

The basic LTE physical resource can be seen as a time-frequency grid.This concept is illustrated in FIG. 2, which shows a number of so-calledsubcarriers in the frequency domain, at a frequency spacing of Δf,divided into OFDM symbol intervals in the time domain. Each individualelement of the resource grid 210 is called a resource element 220, andcorresponds to one subcarrier during one OFDM symbol interval, on agiven antenna port. One aspect of OFDM is that each symbol 230 beginswith a cyclic prefix 240, which is essentially a reproduction of thelast portion of the symbol 230 affixed to the beginning. This featureminimizes problems from multipath, over a wide range of radio signalenvironments.

In the time domain, LTE downlink transmissions are organized into radioframes of ten milliseconds each, each radio frame consisting of tenequally-sized subframes of one millisecond duration. This is illustratedin FIG. 3, where an LTE signal 310 includes several frames 320, each ofwhich is divided into ten subframes 330. Not shown in FIG. 3 is thateach subframe 330 is further divided into two slots, each of which is0.5 milliseconds in duration.

LTE link resources are organized into “resource blocks,” defined astime-frequency blocks with a duration of 0.5 milliseconds, correspondingto one slot, and encompassing a bandwidth of 180 kHz, corresponding to12 contiguous sub-carriers with a spacing of 15 kHz. Resource blocks arenumbered in the frequency domain, starting with 0 from one end of thesystem bandwidth. Two time-consecutive resource blocks represent aresource block pair, and correspond to the time interval upon whichscheduling operates. Of course, the exact definition of a resource blockmay vary between LTE and similar systems, and the inventive methods andapparatus described herein are not limited to the numbers used herein.

In general, however, resource blocks may be dynamically assigned tomobile terminals, and may be assigned independently for the uplink andthe downlink. Depending on a mobile terminal's data throughput needs,the system resources allocated to it may be increased by allocatingresource blocks across several sub-frames, or across several frequencyblocks, or both. Thus, the instantaneous bandwidth allocated to a mobileterminal in a scheduling process may be dynamically adapted to respondto changing conditions.

For scheduling of downlink data, the base station transmits controlinformation in each subframe. This control information identifies themobile terminals to which data is targeted and the resource blocks, inthe current downlink subframe, that are carrying the data for eachterminal. The first one, two, three, or four OFDM symbols in eachsubframe are used to carry this control signaling. In FIG. 4, a downlinksubframe 410 is shown, with three OFDM symbols allocated to controlregion 420. The control region 420 consists primarily of control dataelements 434, but also includes a number of reference symbols 432, usedby the receiving station to measure channel conditions. These referencesymbols 432 are interspersed at pre-determined locations throughout thecontrol region 420 and among the data symbols 436 in the data portion430 of the subframe 410.

Transmissions in LTE are dynamically scheduled in each subframe, wherethe base station transmits downlink assignments/uplink grants to certainmobile terminals (user equipment, or UEs, in 3GPP terminology) via thephysical downlink control channel (PDCCH). The PDCCHs are transmitted inthe control region of the OFDM signal, i.e., in the first OFDM symbol(s)of each subframe, and span all or almost all of the entire systembandwidth. A UE that has decoded a downlink assignment, carried by aPDCCH, knows which resource elements in the subframe that contain dataaimed for that particular UE. Similarly, upon receiving an uplink grant,the UE knows which time-frequency resources it should transmit upon. Inthe LTE downlink, data is carried by the physical downlink sharedchannel (PDSCH) and in the uplink the corresponding channel is referredto as the physical uplink shared channel (PUSCH).

LTE also employs multiple modulation formats, including at least QPSK,16-QAM, and 64-QAM, as well as advanced coding techniques, so that datathroughput may be optimized for any of a variety of signal conditions.Depending on the signal conditions and the desired data rate, a suitablecombination of modulation format, coding scheme, and bandwidth ischosen, generally to maximize the system throughput. Power control isalso employed to ensure acceptable bit error rates while minimizinginterference between cells. In addition, LTE uses a hybrid-ARQ (HARQ)error correction protocol where, after receiving downlink data in asubframe, the terminal attempts to decode it and reports to the basestation whether the decoding was successful (ACK) or not (NACK). In theevent of an unsuccessful decoding attempt, the base station canretransmit the erroneous data.

SUMMARY

In so-called heterogeneous cell deployments, one or more relatively lowpower transmission points are deployed within the coverage area of ahigher power transmission point. A UE that is capable of receiving andprocessing UE-specific reference symbols can receive shared datatransmissions simultaneously from one or more low power transmissionpoints and the high power transmission points, using the UE-specificreference symbols to characterize the transmission channel, and can thusenjoy improved signal reliability and/or higher data rates. However, UEsthat are not configured to process UE-specific reference symbols, andthat instead depend on cell-specific references symbols (CRS) for signaldemodulation, do not directly benefit from this approach. Accordingly,techniques are needed for supporting UEs without UE-specific referencesymbol capabilities in heterogenous cell deployments. More generally,improved techniques are needed for handling cell-common signals, such asCRS and synch channels, in a heterogeneous deployment of transmissionpoints.

In several embodiments of the present invention, the same cell-commonsignals transmitted from the high-power (macro) point in a heterogeneousdeployment, such as CRS and synch channels, are also distributed over atleast some of the low-power (pico) points having coverage areas fallingwithin the coverage area of the high-power point. In some embodiments,the set of low-power points involved in this distribution may be adaptedby the network, based on channel properties of the UEs served by thecell. Similarly, the points participating in transmissions of data,e.g., via a Physical Downlink Shared Channel (PDSCH) and/or a PhysicalDownlink Control Channel (PDCCH) for a particular UE may be the same ordifferent from the set of points used for the CRS distribution,depending on traffic pattern, UE capabilities, and the properties of thechannels for the UE of interest. These techniques permit UEs that relyon CRS for signal demodulation to enjoy improvedsignal-to-interference-plus-noise (SINR) performance when in thevicinity of a low-power point, while maintaining the ability of UEs thatsupport UE-specific reference symbols to enjoy higher data rates.

More particularly, in some embodiments, a high-power point and one ormore low-power points transmit signals, which may be associated to thesame cell-id, in a heterogeneous deployment. The coverage areascorresponding to these low-power points fall within or substantiallywithin the coverage area for the high-power point, so that mobilestations within range of a low-power point are also within range of thehigh-power point, from a downlink perspective.

The same CRS signals are transmitted by both the high-power (macro)point and some or all of the low-power (pico) points. At the same time,the network transmits a physical downlink data channel for a particularUE on both the high-power point as well as on some or all of thelow-power points. In some embodiments, and/or under some circumstances,all of the low-power points participate in the transmission of thephysical downlink data channel to a particular UE that uses CRS forsignal demodulation, while in other embodiments, and/or under somecircumstances, only a subset of the points, e.g., those points that theUE hears sufficiently well, participate in the data transmission to thatUE.

In some embodiments, the network monitors the traffic load of thedifferent points and the channel properties from the points to thevarious UEs. Based on this monitoring, the network can selectively powerdown CRS transmission for points that do not handle a sufficiently largeamount of traffic. In some embodiments this power down/up of CRS is madesmooth, i.e., gradual, over a period including several CRS symboltransmissions, such that the changes in CRS power have a similar rate oftime-variations as the time-variations induced by the channel fading.This approach can help avoid ruining the channel interpolation mechanismon the UE side.

In an example method, such as might be implemented in a network thatincludes a primary transmitting node, having a first coverage area, andone or more secondary transmitting nodes, each having a correspondingcoverage area that falls within the first coverage area, CRS signals aretransmitted from the primary transmitting node. The same CRS signals arealso transmitted from the secondary nodes. A physical downlink sharedchannel (PDSCH) is transmitted from the primary transmitting node aswell as from at least one of the secondary transmitting nodes.

In some cases, of course, there may be more than one secondarytransmitting node in the first coverage area. The same CRS signals aretransmitted from all of the two or more secondary transmitting nodes, insome cases. In some of these cases, the downlink shared channel is alsotransmitted to one or more UEs from all of the two or more secondarytransmitting nodes, while in others it is transmitted from only a subsetof the secondary nodes.

In some cases where there are two or more secondary transmitting nodesin the first coverage area, traffic load at each of the secondarytransmitting nodes is monitored and transmissions of the CRS signal ateach secondary transmitting node are selectively powered on and/or off,based on the corresponding traffic load. This is done, in someinstances, by measuring uplink signals from a plurality of mobilestations in the first coverage area. In some cases, the power levels ofthe transmissions are ramped up and/or ramped down, as appropriate, overa time interval spanning several CRS symbol transmissions, which isselected so that the change in CRS power level is slower than channelvariations due to fading.

In some embodiments, the particular subset of secondary nodes used fortransmitting the physical shared downlink channel is determined based onwhether data transmissions from each secondary transmitting node couldbe received by the mobile station with adequate signal strength. This isdone, in some instances, by measuring uplink transmissions from themobile station at one or more of the secondary transmitting nodes, or bymonitoring channel-state-information feedback from the mobile station,or both.

Apparatus for carrying out the various processes disclosed herein arealso described, including a system of transmitting nodes in a wirelessnetwork as well as a corresponding control unit. Of course, the presentinvention is not limited to the features and advantages summarizedabove. Indeed, those skilled in the art will recognize additionalfeatures and advantages of the present invention upon reading thefollowing detailed description and viewing the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates several pico-cells overlaid by a macro-cell.

FIG. 2 illustrates features of the OFDM time-frequency resource grid.

FIG. 3 illustrates the time-domain structure of an LTE signal.

FIG. 4 illustrates features of an LTE downlink subframe.

FIG. 5 illustrates the mapping of CSI-RS to an LTE resource grid fortwo, four, and eight antenna ports.

FIG. 6 illustrates the differences between uplink and downlink coveragein a mixed cell scenario.

FIG. 7 illustrates the use of inter-cell interference coordination indownlink subframes in a heterogeneous network.

FIG. 8 illustrates a heterogeneous cell deployment where a separatecell-id is used for each point.

FIG. 9 illustrates a heterogeneous cell deployment where the cell-id isshared between the macro-point and pico-points in the macro-point'scoverage area.

FIG. 10 is a process flow diagram illustrating a method for transmittingdata to a mobile station in a heterogeneous cell deployment.

FIG. 11 is a process flow diagram illustrating a method for managingpico-nodes in a heterogeneous cell deployment.

FIG. 12 is a block diagram illustrating features of nodes in aheterogeneous cell deployment.

DETAILED DESCRIPTION

Various embodiments of the present invention are now described withreference to the drawings, where like reference numerals are used torefer to like elements throughout. In the following description,numerous specific details are set forth for purposes of explanation, inorder to provide a thorough understanding of one or more embodiments. Itwill be evident to one of ordinary skill in the art, however, that someembodiments of the present invention may be implemented or practicedwithout one or more of these specific details. In other instances,well-known structures and devices are shown in block diagram form inorder to facilitate describing embodiments.

Note that although terminology from 3GPP's specifications for LTE andLTE-Advanced is used throughout this document to exemplify theinvention, this should not be seen as limiting the scope of theinvention to only these systems. Other wireless systems including oradapted to include heterogeneous cell deployments may also benefit fromexploiting the ideas covered herein.

Demodulation of transmitted data generally requires estimation of theradio channel. In LTE systems, this is done using transmitted referencesymbols (RS), i.e., transmitted symbols having values that are alreadyknown to the receiver. In LTE, cell-specific reference symbols (CRS) aretransmitted in all downlink subframes. In addition to assisting downlinkchannel estimation, the CRS are also used for mobility measurementsperformed by the UEs.

The CRS are generally intended for use by all the mobile terminals inthe coverage area. To support improved channel estimation, especiallywhen multiple-input multiple-output (MIMO) transmission techniques areused, LTE also supports UE-specific reference symbols, which aretargeted to individual mobile terminals and are intended specificallyfor channel estimation for demodulation purposes.

FIG. 4 illustrates how the mapping of physical control/data channels andsignals can be done on resource elements within a downlink subframe 410.In the pictured example, the PDCCHs occupy only the first out of thethree possible OFDM symbols that make up the control region 420, so inthis particular case the mapping of data can begin at the second OFDMsymbol. Since the CRS are common to all UEs in the cell, thetransmission of CRS cannot be easily adapted to suit the needs of aparticular UE. This is in contrast to UE-specific reference symbols, bymeans of which each UE can have reference symbols of its own placed inthe data region 430 of FIG. 4, as part of PDSCH.

The length (one, two, or three symbols) of the control region that isused to carry PDCCH can vary on a subframe-to-subframe basis, and issignaled to the UE in the Physical Control Format Indicator CHannel(PCFICH). The PCFICH is transmitted within the control region, atlocations known by terminals. Once a terminal has decoded the PCFICH, itthen knows the size of the control region and in which OFDM symbol thedata transmission starts.

Also transmitted in the control region is the Physical Hybrid-ARQIndicator Channel. This channel carries ACK/NACK responses to aterminal, to inform the mobile terminal whether the uplink datatransmission in a previous subframe was successfully decoded by the basestation.

As noted above, CRS are not the only reference symbols available in LTE.As of LTE Release-10, a new reference symbol concept was introduced.Separate UE-specific reference symbols for demodulation of PDSCH aresupported in Release 10, as are reference symbols specifically providedfor measuring the channel for the purpose of generating channel stateinformation (CSI) feedback from the UE. The latter reference symbols arereferred to as CSI-RS. CSI-RS are not transmitted in every subframe, andthey are generally sparser in time and frequency than reference symbolsused for demodulation. CSI-RS transmissions may take place every fifth,tenth, twentieth, fortieth, or eightieth subframe, as determined by aperiodicity parameter and a subframe offset, each of which areconfigured by Radio Resource Control (RRC) signaling.

A UE operating in connected mode can be requested by the base station toperform channel state information (CSI) reporting. This reporting caninclude, for example, reporting a suitable rank indicator (RI) and oneor more precoding matrix indices (PMIs), given the observed channelconditions, as well as a channel quality indicator (CQI). Other types ofCSI are also conceivable, including explicit channel feedback andinterference covariance feedback. The CSI feedback assists the basestation in scheduling, including deciding which subframe and resourceblocks to use for the transmission, as well as deciding whichtransmission scheme and/or precoder should be used. The CSI feedbackalso provides information that can be used to determine a proper userbit-rate for the transmission, i.e., for link adaptation.

In LTE, both periodic and aperiodic CSI reporting are supported. In thecase of periodic CSI reporting, the terminal reports the CSImeasurements on a configured periodic time basis, using the physicaluplink control channel (PUCCH). With aperiodic reporting, the CSIfeedback is transmitted on the physical uplink shared channel (PUSCH) atpre-specified time instants after receiving the CSI grant from the basestation. With aperiodic CSI reports, the base station can thus requestCSI that reflects downlink radio conditions in a particular subframe.

A detailed illustration of which resource elements within a resourceblock pair that may potentially be occupied by the new UE-specificreference symbols and CSI-RS is provided in FIG. 5, for the cases inwhich two, four, and eight transmitter antenna ports are used for theCSI transmission. The CSI-RS utilizes an orthogonal cover code of lengthtwo to overlay two antenna ports on two consecutive resource elements.In other words, the CSI-RS are allocated in pairs, where two orthogonalcodes of length two are transmitted simultaneously, using the same pairof allocated resource elements, from a pair of antenna ports at the basestation.

In FIG. 5, the CSI-RS resource elements are designated with numbers,which correspond to antenna port numbers. In the left-hand diagram,corresponding to the case of two CSI-RS antenna ports, the possiblepositions for the CSI-RS are labeled “0” and “1”, corresponding toantenna ports 0 and 1.

As can be seen in FIG. 5, many different CSI-RS pattern are available.For the case of two CSI-RS antenna ports, for instance, where eachCSI-RS pair can be separately configured, there are twenty differentpatterns within a subframe. When there are four CSI-RS antenna ports,the CSI-RS pairs are assigned two at a time; thus the number of possiblepatterns is ten. For the case of eight CSI-RS antenna ports, fivepatterns are available. For TDD mode, some additional CSI-RS patternsare available.

In the following discussion, the term “CSI-RS resource” is used. ACSI-RS resource corresponds to a particular pattern present in aparticular subframe. Thus two different patterns in the same subframeconstitute two distinct CSI-RSI resources. Likewise, the application ofthe same CSI-RS pattern to two different subframes again represents twoseparate instances of a CSI-RS resource, and the two instances are thusagain to be considered distinct CSI-RS resources.

Any of the various CSI-RS patterns pictured in FIG. 5 may alsocorrespond to so-called zero-power CSI-RS, which are also referred to asmuted REs. A zero-power CSI-RS is a CSI-RS pattern whose resourceelements are silent, i.e., there is no transmitted signal on thoseresource elements. These silent patterns are configured with aresolution corresponding to the four-antenna-port CSI-RS patterns.Hence, the smallest unit of silence that may be configured correspondsto four REs.

The purpose of zero-power CSI-RS is to raise thesignal-to-interference-plus-noise ratio (SINR) for CSI-RS in a givencell, by configuring zero-power CSI-RS in interfering cells so that theresource elements that would otherwise cause interference are silent.Thus, a CSI-RS pattern in a given cell is matched with a correspondingzero-power CSI-RS pattern in interfering cells.

Raising the SINR level for CSI-RS measurements is particularly importantin applications such as coordinated multi point (CoMP) or inheterogeneous deployments. In CoMP, the UE is likely to need to measurethe channel from non-serving cells. Interference from the much strongerserving cell would make those measurements difficult, if not impossible.Zero-power CSI-RS are also needed in heterogeneous deployments, wherezero-power CSI-RS in the macro-layer are configured to coincide withCSI-RS transmissions in the pico-layer. This avoids strong interferencefrom macro nodes when UEs measure the channel to a pico-node.

The PDSCH, which carries data targeted for mobile stations, is mappedaround the resource elements occupied by CSI-RS and zero-power CSI-RS,so it is important that both the network and the UE are assuming thesame CSI-RS and zero power CSI-RS configurations. Otherwise, the UE maybe unable to properly decode the PDSCH in subframes that contain CSI-RSor their zero-power counterparts.

The CSI-RS discussed above is used for measurements of the downlinkchannel, i.e., from a base station to a mobile terminal. In the uplink,so-called sounding reference symbols (SRS) may be used for acquiring CSIabout the uplink channel from the UE to a receiving node. When SRS isused, it is transmitted on the last DFT-spread OFDM symbol of asubframe. SRS can be configured for periodic transmission as well fordynamic triggering as part of the uplink grant. The primary use for SRSis to aid the scheduling and link adaptation in the uplink. Fortime-division duplex (TDD) LTE systems, however, SRS is sometimes usedto determine beam-forming weights for the downlink, by exploiting thefact that the downlink and uplink channels are the same when the samecarrier frequency is used for downlink and uplink (channel reciprocity).

While PUSCH carries data in the uplink, PUCCH is used for control. PUCCHis a narrowband channel using a resource block pair where the tworesource blocks are on opposite sides of the potential schedulingbandwidth. PUCCH is used for conveying ACK/NACKs, periodic CSI feedback,and scheduling request to the network.

Before an LTE terminal can communicate with an LTE network it first hasto find and acquire synchronization to a cell within the network, aprocess known as cell search. Next, the UE has to receive and decodesystem information needed to communicate with and operate properlywithin the cell. Finally, the UE can access the cell by means of theso-called random-access procedure.

In order to support mobility, a terminal needs to continuously searchfor, synchronize to, and estimate the reception quality of both itsserving cell and neighbor cells. The reception quality of the neighborcells, in relation to the reception quality of the current cell, is thenevaluated in order to determine whether a handover (for terminals inconnected mode) or cell re-selection (for terminals in idle mode) shouldbe carried out. For terminals in connected mode, the handover decisionis taken by the network, based on measurement reports provided by theterminals. Examples of such reports are reference signal received power(RSRP) and reference signal received quality (RSRQ).

The results of these measurements, which are possibly complemented by aconfigurable offset, can be used in several ways. The UE can, forexample, be connected to the cell with the strongest received power.Alternatively, the UE can be assigned to the cell with the best pathgain. An approach somewhere between these alternatives may be used.

These selection strategies do not always result in the same selectedcell for any given set of circumstances, since the base station outputpowers of cells of different type are different. This is sometimesreferred to as link imbalance. For example, the output power of a picobase station or a relay node is often on the order of 30 dBm (1 watt) orless, while a macro base station can have an output power of 46 dBm (40watts). Consequently, even in the proximity of the pico cell, thedownlink signal strength from the macro cell can be larger than that ofthe pico cell. From a downlink perspective, it is often better to selecta cell based on downlink received power, whereas from an uplinkperspective, it would be better to select a cell based on the path loss.

These alternative cell selection approaches are illustrated in FIG. 6.The solid lines emanating from each of macro-cell 110 and pico-cell 120represent the received power at each point between the two cells. Theselines intersect, i.e., are equal, at border 540. Accordingly, a UEwithin region 510 will see a stronger received signal from the pico-cell120, and will get the best downlink performance if it selects pico-cell120. The dashed lines issuing from pico-cell 120 and macro-cell 110, onthe other hand, represent the path loss between a UE at a given pointand either the macro-cell 110 or the pico-cell 120. Because the pathloss is not weighted by the transmitter output power, these linesintersect at a point halfway between macro-cell 110 and pico-cell 120,as seen at the boundary 530. A UE outside region 520, then, willexperience a lower path loss to macro-cell 110 than to pico-cell 120,and will thus achieve better uplink performance if it selects macro-cell110. Because of this unbalanced situation, there is a region, i.e., theportion of coverage area 520 that is outside coverage area 510, in whichneither cell is optimal for both downlink and uplink performance at thesame time.

From a system perspective, it might often be better, in the abovescenario, for a given UE to connect to the pico-cell 120 even under somecircumstances where the downlink from macro-cell 110 is much strongerthan the pico cell downlink. However, ICIC across layers will be neededwhen the terminal operates within the region between the uplink anddownlink borders, i.e., the link imbalance zone, as depicted in FIG. 6.

Interference coordination across the cell layers is especially importantfor the downlink control signaling. If the interference is not handledappropriately, a terminal that is in the region between the downlink anduplink borders in FIG. 6 and is connected to pico-cell 120 may be unableto receive the downlink control signaling from the pico-cell 120.

One approach to providing ICIC across layers is illustrated in FIG. 7.An interfering macro-cell, which could create downlink interferencetowards a pico-cell, transmits a series of subframes 710, but avoidsscheduling unicast traffic in certain subframes 712. In other words,neither PDCCHs nor PDSCH are transmitted in those subframes 712. In thisway, it is possible to create low-interference subframes, which can beused to protect users of the pico-cell who are operating in the linkimbalance zone.

To carry out this approach, the macro-base station (MeNB) indicates tothe pico-base station (PeNB), via the backhaul interface X2, whichsubframes will not be used to schedule users. The PeNB can then takethis information into account when scheduling users operating within thelink imbalance zone, such that these users are scheduled only insubframes 722 aligned with the low-interference subframes transmitted inthe macro layer. In other words, these users are scheduled only ininterference-protected subframes. Pico-cell users operating within thedownlink border, e.g., within coverage area 510 in FIG. 6, can bescheduled in all subframes, i.e., in both the protected subframes 722 aswell as the remaining, un-protected, subframes in the series ofsubframes 720.

In principle, data transmission (but not control signaling) in differentlayers could also be separated in the frequency domain by ensuring thatscheduling decisions in the two cell layers are non-overlapping in thefrequency domain. This could be facilitated by exchanging coordinationmessages between the different base stations. However, this is notpossible for the control signaling, since the control signaling spansthe full bandwidth of the signal, according to the LTE specifications,and hence a time-domain approach must be used.

The classical way of deploying a network is for each differenttransmission/reception point to provide coverage for a cell that isdistinct from all others. That is, the signals transmitted from orreceived at one point are associated with a cell identifier (cell-id)that is different from the cell-id employed for other nearby points.Typically, each of these points transmits its own unique signals forbroadcast, e.g., the Physical Broadcast Channel (PBCH), as well as forsync channels, such as the primary synchronization signal (PSS) andsecondary synchronization signal (SSS).

The concept of a “point” is heavily used in conjunction with techniquesfor coordinated multipoint (CoMP). In this context, a point correspondsto a set of antennas covering essentially the same geographical area ina similar manner. Thus, a point might correspond to one of the sectorsat a site, but it may also correspond to a site having one or moreantennas all intending to cover a similar geographical area. Often,different points represent different sites. Antennas correspond todifferent points when they are sufficiently geographically separatedand/or have antenna diagrams pointing in sufficiently differentdirections. Techniques for CoMP entail introducing dependencies in thescheduling or transmission/reception among different points, in contrastto conventional cellular systems where a point is operated more or lessindependently from the other points, from a scheduling point of view.

The classical strategy of one cell-id per point is depicted in FIG. 8for a heterogeneous deployment where a number of low-power (pico) points120 are placed within the coverage area of a higher power macro point110. In this deployment, the pico-nodes transmit different cellidentifiers, i.e., “cell-id 2”, “cell-id 3”, and “cell-id 4”, from thecell identifier “cell-id 1” transmitted by the macro-cell 110. Note thatsimilar principles obviously also apply to classical macro-cellulardeployments where all points have similar output power and perhaps areplaced in a more regular fashion than what is the case for aheterogeneous deployment.

An alternative to the classical deployment strategy is to instead letall the UEs within a geographical area outlined by the coverage of thehigh power macro point be served with signals associated with the samecell-id. In other words, from a UE perspective, the received signalsappear as though they come from a single cell. This is illustrated inFIG. 9. Here, all of the pico-nodes 120 transmit the same cellidentifier, “cell-id 1”, which is also used by the overlaying macro-cell110.

Note that in both FIGS. 8 and 9 only one macro point is shown; othermacro points would typically use different cell-ids (corresponding todifferent cells) unless they are co-located at the same site(corresponding to other sectors of the macro site). In the latter caseof several co-located macro points, the same cell-id may be sharedacross the co-located macro-points and those pico points that correspondto the union of the coverage areas of the macro points. Sync, BCH andcontrol channels are all transmitted from the high-power point whiledata can be transmitted to a UE also from low-power points by usingshared data transmissions (PDSCH) that rely on UE-specific referencesymbols.

Such an approach has benefits for those UEs that are capable ofreceiving PDSCH based on UE-specific reference symbols, while UEs thatonly support CRS for PDSCH have to settle for using only thetransmission from the high-power point, and thus will not benefit in thedownlink from the deployment of extra low-power points. This lattergroup is likely to include at least all Release 8 and 9 UEs for use inFDD LTE systems.

The single cell-id approach for heterogeneous and/or hierarchical celldeployments is geared towards situations in which there is fast backhaulcommunication between the points associated with the same cellidentifier. A typical case would be a base station serving one or moresectors on a macro level as well as having fast fiber connections toremote radio units (RRUs) performing the role of the other points thatshare the same cell-id. Those RRUs could represent low-power points withone or more antennas each. Another example is when all the points have asimilar power class, with no single point having more significance thanthe others. The base station would then handle the signals from all RRUsin a similar manner.

A clear advantage of the shared cell-id approach compared with theclassical one is that the handover procedure between cells only needs tobe invoked on a macro basis. Another important advantage is thatinterference from CRS can be greatly reduced, since CRS does not have tobe transmitted from every point. There is also much greater flexibilityin coordination and scheduling among the points, which means the networkcan avoid relying on the inflexible concept of semi-staticallyconfigured low-interference subframes, as illustrated in FIG. 7. Ashared-cell approach also allows decoupling of the downlink from theuplink, so that, for example, path-loss-based reception-point selectioncan be performed for the uplink, without creating a severe interferenceproblem for the downlink, where the UE may be served by a transmissionpoint different from the point used in the uplink receptions.

One problem with existing solutions for shared-cell-id deployments isthe handling of UEs that do not support UE-specific reference symbols.Those UEs use CRS, and do not directly benefit, from a downlinkperspective, from the deployment of extra low-power points in aheterogeneous deployment. Another potential problem is that the coverageof synch channels may also be limited, for all UEs.

Accordingly, in several embodiments of the present invention, the samecell-common signals transmitted from the high-power (macro) point in aheterogeneous deployment, such as CRS and synch channels, are alsodistributed over at least some of the low-power (pico) points associatedwith the same cell. In some embodiments, the set of low-power pointsinvolved in this distribution may be adapted by the network, based onchannel properties of the UEs served by the cell. Similarly, the pointsparticipating in CRS-based transmissions of the data of PDSCH and/orPDCCH for a particular UE may be the same or different from the set ofpoints used for the CRS distribution, depending on traffic pattern, UEcapabilities, and the properties of the channels for the UE of interest.

Note that this high-level sketch of example features of some embodimentsof the invention does not include the distribution of synch channel andhow that may be different from how CRS is transmitted. Those skilled inthe art will appreciate that similar approaches may be taken for thesynch channel, but also that these approaches may be adapted to accountfor the different ways in which the synch channel is transmitted andused.

More particularly, in some embodiments, a high-power point and one ormore low-power points transmit signals associated to the same cell-id,in a heterogeneous deployment. The coverage areas corresponding to thelow-power points fall at least partly within the coverage area for thehigh-power point, so that mobile stations within range of a low-powerpoint are also within range of the high-power point (from a downlinkperspective).

The same CRS signals are transmitted by both the high-power (macro)point and some or all of the low-power (pico) points. At the same time,the network transmits CRS-based PDSCH for a particular UE on both thehigh-power point as well as on some or all of the low-power points. Insome embodiments, and/or under some circumstances, all of the low-powerpoints participate in the CRS-based PDSCH transmission to a particularUE, while in other embodiments, and/or under some circumstances, only asubset of the points, e.g., those points that the UE hears sufficientlywell, participate in the PDSCH transmission using CRS for channelestimation.

In some embodiments, the network monitors the traffic load of thedifferent points and the channel properties from the points to thevarious UEs. Based on this monitoring, the network can selectively powerdown CRS transmission for points that do not handle a sufficiently largeamount of traffic. It may be advantageous in some embodiments to makethis power down/up of CRS smooth (i.e., gradual), such that the changesin CRS power have a similar rate of time-variations as thetime-variations induced by the channel fading. This approach can helpavoid ruining the channel interpolation mechanism on the UE side.

To improve the performance for all UEs when adding low-power points to ahigh-power point in a heterogeneous shared cell-id scenario, the sameCRS can be transmitted from both the high-power as well as low-powerpoints, to achieve combining gains at the receiving UEs. The sameapplies to PDSCH transmissions for which the UEs rely on CRS fordemodulation; these transmissions are referred to herein as “CRS-based”PDSCH transmissions. Thus, at least some of the low-power points sharingthe same cell-id also participate in the CRS-based PDSCH transmission toa given UE that uses CRS for PDSCH. In one example, all pointsassociated with the same cell-id transmit CRS and PDSCH based on CRS.CRS are also used for PBCH, PDCCH, PHICH, and PCFICH, so these channelscan also be distributed over a mixture of high- and low-power points.

The sync channels PSS and SSS represent antenna ports of their own andmay be transmitted in a different manner than the CRS. Nevertheless, insome embodiments PSS and SSS also are transmitted from all the pointsoperated by the same base station.

Distributing the signals in this manner helps UEs that do not supportUE-specific reference symbols to benefit (in the downlink) from theadditional low-power points, by raising their SINR levels when they arein the vicinity of a low-power point. At the same time, UEs that dosupport UE-specific reference symbols can continue enjoying extracapacity gains, since the same time-frequency transmission resources canbe used by different points, for downlink transmissions to differentUEs, as long as they are sufficiently isolated from each other.

An alternative to letting CRS-based PDSCH be transmitted from all pointsis to only use a subset of the points for such a transmission, even ifthe CRS is transmitted from all or another subset of points. In otherwords, CRS and CRS-based PDSCH need not be transmitted from preciselythe same set of low-power points. To do this, it is advantageous ifPDSCH is transmitted from at least all the points the UE hearssufficiently well. Otherwise, channel estimates based on CRS would nolonger adequately reflect the channel that the data is conveyed over.For example, if the UE is close to one of the low-power points, data aspart of PDSCH might only be transmitted from that point, while the UEwould still estimate the channel based on CRS coming from all points.

Points that serve no UEs or that are not crucial in serving any(CRS-dependent) UEs could be powered down, either completely or in part,for example by turning off the CRS transmission. When doing this, it maybe beneficial in some systems or in some circumstances to power down theCRS in a smooth non-abrupt fashion, so as to not distort the channelestimates in the UE, which typically uses interpolation in time to formchannel estimates. Based on the traffic load and the properties of thechannels from the points to the UEs, the network can select the pointsin which to power down CRS and associated signals. As previouslymentioned, some UEs only support reception of CRS-based transmissions,so capabilities of the served UEs may be another factor to take intoaccount when deciding on what points to power down. Usually, the CRStransmissions from the high-power (macro) point would not be powereddown, since these transmissions offer a kind of baseline coverage withinthe macro coverage area. However, circumstances in which CRStransmissions from the macro point may be powered down or off areconceivable, such as when all the UEs in the macro cell are best serveddirectly by low-power points.

Channel properties used in deciding what points to power down may beacquired from measurements of signals in the uplink. For instance,measurements on SRS, PUCCH, or PUSCH may give an indication of the pathloss between a point and a UE. CSI feedback from the UE may also beused. Naturally, there is a corresponding power-up procedure working ina similar manner to the above-described power-down procedure; thispower-up procedure may also be based on measurements of uplink signalsand/or UE CSI feedback.

A similar power-down/up mechanism can be applied to other transmissionsignals including the synchronization signals PSS and SSS. Smoothness inpower-down/up may be achieved by gradually increasing the rate at whichthe power is reduced or increased, followed by gradually decreasing therate of change as the power approaches the intended new level. The rateat which the power changes may depend on the rate of time-variationsinduced by the channel fading, so as to not upset possible time-domaininterpolation algorithms in the UEs.

Embodiments of the above-described techniques include methods performedat one or more nodes in a network (e.g, at an LTE eNodeB) forconfiguring the transmission of CRS and/or one or more other cell-commonsignals, such as synch signals. In several embodiments, a subset oflow-power points associated with a given cell are selected andconfigured to transmit the same CRS as the high-power point in the cell;in some embodiments, the same subset or a different subset are selectedand configured to transmit one or more cell-common synch signals. ACRS-based physical downlink shared channel (PDSCH) is simultaneouslytransmitted from the high-power cell and at least one of the low-powerpoints that are transmitting the CRS.

In some embodiments, these selections are based on one or more ofchannel conditions between low-power points and one or more UEs; theseselections may also be based on UE capabilities (e.g., lack of supportfor UE-specific reference symbols) and/or UE CSI feedback. One or moreof these methods may be based on measurement data received from themobile terminals, including CSI feedback, and may alternatively and/oralso depend on identification by the mobile terminals of well-heardtransmission points.

FIG. 10 illustrates a process flow diagram according to severalembodiments of the present invention, such as might be implemented in anetwork that includes a primary transmitting node, having a firstcoverage area, and one or more secondary transmitting nodes, each havinga corresponding coverage area that falls within the first coverage area.The illustrated process includes, as shown at block 1010, thetransmission of CRS signals from the primary, or “macro” node. Thetechnique also includes the transmission of the same CRS signals fromthe secondary, or “pico” nodes, as shown at block 1020. Finally, asshown at block 1030, the illustrated process also includes thetransmission of a physical downlink shared channel from the primarytransmitting node as well as from at least one of the secondarytransmitting nodes.

In some cases, of course, there may be more than one secondarytransmitting node in the first coverage area. In some cases, the sameCRS signals are transmitted from all of the two or more secondarytransmitting nodes. In some of these cases, the CRS-based physicaldownlink shared channel is also transmitted from all of the two or moresecondary transmitting nodes, while in others it is transmitted fromonly a subset of the secondary nodes.

In some cases where there are two or more secondary transmitting nodesin the first coverage area, traffic load at each of the secondarytransmitting nodes is monitored and transmissions of the CRS signal ateach secondary transmitting node are selectively powered on and/or off,based on the corresponding traffic load. This is done, in someinstances, by measuring uplink signals from a plurality of mobilestations in the first coverage area. This is shown in FIG. 11, at blocks1110 and 1120, respectively. In some cases, the power levels of thetransmissions are ramped up and/or ramped down, as appropriate, over atime interval that is selected so that the change in CRS power level isslower than channel variations due to fading.

In some embodiments of the process illustrated in FIG. 10, theparticular subset of secondary nodes used for transmitting the physicalshared downlink channel is determined based on whether PDSCHtransmissions from each secondary transmitting node could be received bythe mobile station with adequate signal strength. This is done, in someinstances, by measuring uplink transmissions from the mobile station atone or more of the secondary transmitting nodes, or by monitoringchannel-state-information feedback from the mobile station, or both.This process is illustrated at blocks 1130 and 1140 of FIG. 11.

Other embodiments of the inventive techniques disclosed herein include awireless system, including a primary node and one or more secondarynodes, corresponding to the methods and techniques described above. Insome cases, the methods/techniques described above will be implementedin a system of transmitting nodes such as the one pictured in detail inFIG. 12.

The system pictured in FIG. 12 includes a macro node 110, two pico nodes120, a UE 130, and an O&M node 190. The macro node 110 is configured tocommunicate with pico nodes 120 and O&M node 190 via inter-base-stationinterface 1204, which comprises suitable network interface hardwarecontrolled by software carrying out network interfacing protocols. Macronode 110 includes a receiver 1202 and transmitter 1206 for communicatingwith UE 130; in some cases receiver 1202 may also be configured tomonitor and/or measure signals transmitted by pico node 120. Receivercircuit 1202 and transmitter circuit 1206 use known radio processing andsignal processing components and techniques, typically according to aparticular telecommunications standard such as the 3GPP standard forLTE-Advanced. Because the various details and engineering tradeoffsassociated with the design of interface circuitry and radio transceivercircuits are well known and are unnecessary to a full understanding ofthe invention, additional details are not shown here.

Macro node 110 further includes a processing circuit 1210, whichincludes one or more microprocessors or microcontrollers, as well asother digital hardware, which may include digital signal processors(DSPs), special-purpose digital logic, and the like. Either or both ofthe microprocessor(s) and the digital hardware may be configured toexecute program code stored in memory 1220, along with stored radioparameters. Again, because the various details and engineering tradeoffsassociated with the design of baseband processing circuitry for mobiledevices and wireless base stations are well known and are unnecessary toa full understanding of the invention, additional details are not shownhere. However, several functional aspects of the processing circuit 1210are shown, including a measuring unit 1212, a control unit 1214, and aconfiguration unit 1216. Configuration unit 216 controls radiotransmitter 1206 to transmit CRS and PDSCH, under the control of controlunit 1214, which also manages the communications with other nodes viainter-BS interface circuit 1204. Control unit 1214 also evaluates dataobtained from measuring unit 1212, such as channel state informationand/or load information, and controls inter-base-station communicationand transmitter configuration accordingly.

Program code stored in memory circuit 1220, which may comprise one orseveral types of memory such as read-only memory (ROM), random-accessmemory, cache memory, flash memory devices, optical storage devices,etc., includes program instructions for executing one or moretelecommunications and/or data communications protocols, as well asinstructions for carrying out one or more of the techniques describedherein, in several embodiments. Radio parameters stored in memory 1220may include one or more pre-determined tables or other data forsupporting these techniques, in some embodiments.

Pico nodes 120 may comprise components and functional blocks verysimilar to those illustrated in macro node 110, with the correspondingcontrol units being responsible for receiving control instructions froma macro node 110 (or other pico node 120) and configuring the piconode's transmitter circuits accordingly.

Implementations of the inventive techniques described herein allow evenUEs that only support CRS-based demodulation to benefit from increasedperformance in the downlink when low-power transmission points aredeployed. Combining of the signals from several transmission nodes occurin the air, resulting in increased SINR levels.

CRS may be transmitted from a multitude of transmission points withoutcreating severe interference problems for UEs that are served by alow-power point while also receiving a stronger signal from a high powerpoint. Similar benefits are present for the synch channels. Furtherreduction of interference as well as energy savings are possible byemploying the described power-down/up procedures while making sure thatchannel interpolation in the UEs does not ruin the channel estimate andthus the demodulation performance.

Examples of several embodiments of the present invention have beendescribed in detail above, with reference to the attached illustrationsof specific embodiments. Because it is not possible, of course, todescribe every conceivable combination of components or techniques,those skilled in the art will appreciate that the present invention canbe implemented in other ways than those specifically set forth herein,without departing from essential characteristics of the invention. Thepresent embodiments are thus to be considered in all respects asillustrative and not restrictive.

What is claimed is:
 1. A method for transmitting data to a mobilestation, the method comprising transmitting cell-specific referencesymbol (CRS) signals from a primary, high-power, transmitting nodehaving a first coverage area; transmitting the same CRS signals fromeach of two or more secondary, low-power, transmitting nodes, each ofthe secondary transmitting nodes having a coverage area that is withinor substantially within the first coverage area; and simultaneouslytransmitting a physical downlink data channel to a mobile station fromthe primary, high-power, transmitting node and from at least one of butfewer than all of the two or more secondary, low-power, transmittingnodes, wherein said at least one but fewer than all of the two or moresecondary, low-power, transmitting nodes is selected based on adetermination of whether the physical downlink data channeltransmissions from each secondary transmitting node could be received bythe mobile station with adequate signal strength.
 2. The method of claim1, wherein said determination is made by measuring uplink transmissionsfrom the mobile station at one or more of the secondary transmittingnodes.
 3. The method of claim 1, further comprising transmitting thesame synchronization channel from all of said secondary transmittingnodes.
 4. A system of transmitting nodes in a wireless network, thesystem comprising: a primary, high-power, transmitting node configuredto transmit cell-specific reference symbol (CRS) signals over a firstcoverage area, and; two or more secondary, low-power, transmittingnodes, each configured to transmit the same CRS signals over a coveragearea that is within or substantially within the first coverage area;wherein said primary transmitting node and at least one of but fewerthan all of said two or more secondary transmitting nodes are configuredto simultaneously transmit a physical downlink data channel to a firstmobile station; wherein the system further comprises a control elementconfigured to select said at least one but fewer than all of the two ormore secondary transmitting nodes for transmitting the physical downlinkdata channel to the mobile station, based on whether physical downlinkdata channel transmissions from each secondary transmitting node couldbe received by the mobile station with adequate signal strength.
 5. Thesystem of claim 4, wherein the control element is configured to selectsaid at least one but fewer than all of the two or more secondarytransmitting nodes based on measurements of uplink transmissions fromthe mobile station at one or more of the secondary transmitting nodes.6. The system of claim 4, wherein all of said two or more secondarytransmitting nodes are configured to transmit the same synchronizationsignal.
 7. A control unit for use in a wireless network having aprimary, high-power, transmitting node with a first coverage area and aplurality of secondary, low-power, transmitting nodes, each having acoverage area that is within or substantially within the first coveragearea, the control unit comprising: a network communication circuitconfigured to transmit and receive control information to and from saidsecondary transmitting nodes; and a processing circuit configured toconfigure said primary transmitting node to transmit cell-specificreference symbol (CRS) signals over the first coverage area; wherein theprocessing circuit is further configured to: configure two or more ofsaid secondary transmitting nodes to transmit the same CRS signals; andconfigure said primary transmitting node and at least one of but fewerthan all of said two or more secondary transmitting nodes tosimultaneously transmit a physical downlink data channel to a mobilestation, wherein the processing circuit is further configured to selectsaid at least one but fewer than all of said two or more secondarytransmitting nodes based on a determination of whether physical downlinkdata channel transmissions from each secondary transmitting node couldbe received by the mobile station with adequate signal strength.
 8. Thecontrol unit of claim 7, wherein the processing circuit is configured toconfigure all of said secondary transmitting nodes in the first coveragearea to transmit the same CRS signals.
 9. The control unit of claim 7,wherein the processing circuit is configured to select said at least onebut fewer than all of said two or more secondary transmitting nodesbased on measurements of uplink transmissions from the mobile station atone or more of the secondary transmitting nodes.
 10. The control unit ofclaim 7, wherein the control unit is part of the primary transmittingnode.
 11. The control unit of claim 7, wherein the control unit is partof one of the secondary transmitting nodes, and wherein the networkcommunication circuit is further configured to transmit and receivecontrol information to and from the primary transmitting node.
 12. Amethod implemented by a control unit for use in a wireless networkcomprising a primary, high-power, transmitting node having a firstcoverage area and a plurality of secondary, low-power, transmittingnodes, each having a coverage area that is within or substantiallywithin the first coverage area, the method comprising: configuring saidprimary transmitting node to transmit cell-specific reference symbol(CRS) signals over the first coverage area; configuring two or more ofsaid secondary transmitting nodes to transmit the same CRS signals;configuring the primary transmitting node and at least one of but fewerthan all of the two or more secondary transmitting nodes tosimultaneously transmit a physical downlink data channel to a mobilestation; selecting said at least one but fewer than all of the two ormore secondary transmitting nodes based on a determination of whetherphysical downlink data channel transmissions from each secondarytransmitting node could be received by the mobile station with adequatesignal strength.